1^28 



EXPERIMENTAL CHICK EMBRYOLOGY 



maJtlng an ever-increaslngly large circular hole. It will help to limit the shell cracking 

 If a small hack saw is used to cut a shallow ring around the shell, Just within the limit 

 of the air space. The shell should be picked away to within about ^ inch of the inner 

 shell membrane and embryo. Pick away all of the outer shell membrane, but remember that 

 this membrane extends completely around the embryo. Avoid ripping or tearing this tough 

 membretne . 



Place a sterile shell cap over the exposed embryo, see that the egg is securely held 

 in the Permoplast of the Syracuse dish, and return the egg to the incubator. The humidity 

 of the incubator must be increased to above 6o^ for these eggs, since there is a much 

 greater exposed surface for evaporation than in the other method. 'B^ the 17th day the egg 

 shell may be sprinkled twice daily with a small amount of sterile water at the incubator 

 temperature, but do not immerse the egg in water. The embryo can be drowned within the 

 shell. 



MORPHOGENETIC MOVEMENTS AS DETERMINED BY VITAL 

 STAINING AND CHARCOAL PARTICLES 



Before beginning the following study of morphogenetlc movements the student should 

 re-acquaint himself with the various descriptions of the processes of normal development 

 from the earliest primitive streak stage (about l6 hours of incubation) to at least U2 

 hours of Incubation when the heart- starts to beat. (See Lillie: "The Development of the 

 Chick".) The pre-gastrulation stages are not available, since they occur within the ovi- 

 duct and every fertilized egg is at least in the primitive streak stage when layed. A 

 condensed survey la given below. 



The primitive streak, considered by many 

 as homologous to the blastoporal lips of the 

 amphibia, consists of a longitudinal thicken- 

 ing of ectoderm extending through from almost 

 the anterior liriit throi;igh about 2/5 of the 

 length of the area pellucida. The primitive 

 groove is probably formed as a result of 

 mesodermal outgrowth, and beneath it all is 

 a thin layer of endoderm lying on the yolk 

 and attached to the streak only at the level 

 of Hensen's node. The bulk of the embryonic 

 tissues arise from material of this streak 

 as it becomes telescoped posteriorly in favor 

 of an anteriorly elongating embryo. The mar- 

 gins of the area pellucida and the area opaca 

 together fonn the extra-embryonic structures. 



PRIMITIVE 

 STR£«K 



Presumptive areas of the chick primitive 

 streak, modified from Wllller & Rawles 

 1935: Proc. Soc. Exp. Biol. & Med. 

 32:1293. 



At about 20 hours of incubation a head 

 process appears anterior to the primitive 

 streak, and this consists of the anterior limit of the notochord and overlying medullary 

 plate ectoderm. There is definite cephallzation (precocious development of the anterior 

 structures) but as the embryo lengthens the primitive streak shortens (with the recession 

 of its anterior end, or Hensen's node). As the neural folds of the future brain region 

 become approximated, the medullary plate is lengthened posteriorly and the somites begin 

 to appear, formed out of mesenchyme which was derived (by migration) from the sides of the 

 primitive streak. The first pair of somites appear at about 21 hours of incubation and 

 will be located at a position Just posterior to the future otic vesicles. The first four 

 somite pairs appear during the first day, all to be Incorporated in the head (occipital) 

 musculature. The primitive streak is therefore not to be considered as part of the embryo 

 proper, but rather as a remnant of the blastoporal lips out of which are derived the tis- 

 sues of the embryo. 



Wetzel (1929) and Pasteels (1957) have mapped out the prospective organ-forming areas 

 of the primitive streak stage of the chick embryo, in a manner similar to that used by 

 Vogt (1926) eind others on the amphibia, by the xxae of vital dyes. In fact, these investi- 

 gators have found that there is surfirlslng similarity in amphibian and avian morphogenetlc 



